How Temperature Changes Solder Your PCB

​Understanding Solder Paste Reflow Stages​

When solder paste heats up during reflow, the process goes through five distinct stages. This article focuses on two key aspects:

1. The traditional temperature profile (with ramp-up, holding, and reflow phases)
2. The simpler RTS (Ramp-to-Spike) temperature curve

How Ramp-to-Spike (RTS) Profiles Solve Heating Problems

Older reflow ovens often heat circuit boards unevenly. Here’s why this happens and how RTS helps:

​​The Problem:​​

• Different colored/textured components and PCB layers absorb heat at different rates
• This creates hot and cold spots across the board (called ΔT)
• Large ΔT causes:
  • Some areas getting too hot (risking burnt parts or residues)
  • Other areas staying too cold (leading to poor solder joints)
• Resulting defects include:
  • Solder balls
  • Poor wetting
  • Component damage
  • Voids in solder
  • Burnt flux residue

How RTS Helps:​​
The Ramp-to-Spike profile provides more controlled, even heating that minimizes these ΔT issues.

Why We Use a Holding Temperature (And When We Don’t Need It)​​

The holding stage in reflow soldering has just one job: to prevent big temperature differences (ΔT) across the board. Here’s how it works:

1. ​​What it does:​​
   • Evens out temperatures across all components
   • Ensures everything reaches melting point together
   • Helps all solder joints form at the same time
2. ​​What you should know:​​
   • The holding stage isn’t always necessary
   • Many boards can use a simpler Ramp-to-Spike (RTS) profile instead
   • Contrary to popular belief, the holding stage doesn’t “activate” flux
   • Modern solder pastes work perfectly well without it

​​Bonus Benefit:​​
RTS profiles often give better results than traditional profiles with holding stages. They typically provide:

• Better solder wetting
• Fewer defects
• More consistent joints

​​Key Takeaways:​​

You might get better results by simplifying your temperature profile

Holding stages help with temperature balance, but aren’t essential

RTS profiles work well with today’s solder pastes

Understanding Ramp-Soak-Reflow (RSS) vs. Ramp-to-Spike (RTS) Profiles​​

RSS Profile Basics:​​

• Works with RMA and no-clean solder pastes
• Not recommended for water-soluble pastes (can damage activators)
• Main purpose: Reduces temperature differences (ΔT) across the board

​​Typical RSS Process:​​

1. Initial heating:
   • 150°C rise in 90 seconds (max 2-3°C/sec)
2. Soak stage:
   • 90 seconds at 150-170°C (balances temperatures)
3. Reflow:
   • 60±15 seconds above 183°C
4. Complete cycle:
   • 3.5-4 minutes total (45°C to 215±5°C peak)
5. Cooling:
   • Optimal at 4°C/sec (faster cooling = stronger joints)
   • Warning: Cooling too fast causes thermal shock

Why RTS is Often Better:​​

Works with ALL pastes and alloys

Especially good for: Water-soluble pastes, Hard-to-solder materials.

Key advantages:

• Brighter solder joints
• Fewer defects
• Better wetting
• More energy efficient
• Easier to adjust

Technical Benefits of RTS:​​

• Controlled heating prevents thermal shock
• Flux stays active longer during preheat
• Uses less oven energy
• Simpler troubleshooting than RSS

​​Recommendation:​​
While RSS still has its uses, RTS generally provides better results with fewer complications – especially for challenging soldering applications.

How to Set Up an RTS Temperature Profile​​

What is an RTS Profile?​​
An RTS (Ramp-to-Spike) profile is a continuous heating process that takes your assembly from room temperature directly to reflow temperature. Here’s how to set it up properly:

​​Preheat Stage (First Two-Thirds of Curve)​​

• Temperature range: Below 150°C
• Heating rate: 0.6-1.8°C per second
• What happens during this stage:
  ✓ Flux activates
  ✓ Moisture evaporates
  ✓ Board prepares for reflow
  ✓ Prevents thermal shock
• Pro tip: Keep the first 90 seconds as steady as possible

​​Why Keep Temperatures Low Initially?​​

• Solder paste activators work best below 150°C
• Staying cooler longer means:
  ✓ Better activator performance
  ✓ Improved solder wetting
  ✓ Shinier solder joints

​​Reflow Stage (Final One-Third)​​

1. After reaching 150°C:
   • Heat quickly to peak temperature (215±5°C)
2. Maintain liquid state for 60±15 seconds to:
   ✓ Reduce flux traps
   ✓ Minimize voids
   ✓ Increase joint strength

​​Complete Profile Specifications​​

• Total time: 3.5-4 minutes (room temp to peak)
• Cooling rate: 4°C/second (optimal for strong joints)
• Warning: Don’t exceed 4°C/second cooling to avoid thermal stress

​​Key Benefits of RTS:​​

• More reliable than traditional profiles
• Better for modern solder pastes
• Easier to control
• Produces higher quality joints

This is my personal interpretation. If you have any questions, please feel free to point them out.